I built a Ramsey Kit cell phone device to decode the TELCO touch tones. This box drove high power 110 VAC relays for engine heater, cabin heater, hangar lights, etc. The trick was to find a cell phone with a headset jack (NOT Bluetooth) to connect the audio to the "box" and that would "auto answer". The cell phone is "float-charged" by a clock timer so the battery stays up. The only on-going expense is a "pay as you go" cell phone SIM chip. I have had it for many years and recently upgraded to a "hot-spot" thus allowing WIFI connectivity to WEMO switches accomplishing the tasks I mentioned above. More expensive than the cell phone but allows me to monitor WIFI connected web cams for inside the hangar and outside (to see how much snow I will have to shovel!!).